Periodontal diseases are bacterial-associated inflammatory diseases of the supporting tissues of the teeth and range from the relatively mild form of gingivitis, the non-specific, reversible inflammation of gingival tissue to the more aggressive forms of periodontitis which are characterised by the destruction of the tooth's supporting structures. Periodontitis is associated with a subgingival infection of a consortium of specific Gram-negative bacteria that leads to the destruction of the periodontium and is a major public health problem. One bacterium that has attracted considerable interest is Porphyromonas gingivalis as the recovery of this microorganism from adult periodontitis lesions can be up to 50% of the subgingival anaerobically cultivable flora, whereas P. gingivalis rarely recovered, and then in low numbers, from healthy sites. A proportional increase in the level of P. gingivalis in subgingival plaque has been associated with an increased severity of periodontitis and eradication of the microorganism from the cultivable subgingival microbial population is accompanied by resolution of the disease. The progression of periodontitis lesions in non-human primates has been demonstrated with the subgingival implantation of P. gingivalis. These findings in both animals and humans suggest a major role for P. gingivalis in the development of adult periodontitis.
P. gingivalis is a black pigmented, anaerobic, proteolytic Gram-negative rod that obtains energy from the metabolism of specific amino acids. The microorganism has an absolute growth requirement for iron, preferentially in the form of heme or its Fe(III) oxidation product hemin and when grown under conditions of excess hemin is highly virulent in experimental animals. A number of virulence factors have been implicated in the pathogenicity of P. gingivalis including the capsule, adhesins, cytotoxins and extracellular hydrolytic enzymes. In order to develop an efficacious and safe vaccine to prevent P. gingivalis colonisation it is necessary to identify effective antigens that are involved in virulence that have utility as immunogens to generate neutralising antibodies.
We have purified and characterised a 300 kDa multiprotein complex of cysteine proteinases and adhesins which is a major virulence factor for P. gingivalis. This complex was biochemically characterised and disclosed in International Patent Application No. PC/AU96/00673, the disclosure of which is incorporated herein by reference. The complex consists of a 160 kDa Arg-specific proteinase with C-terminal adhesin domains (designated PrtR) associated with a 163 kDa Lys-specific proteinase also with C-terminal adhesin domains (designated PrtK). The C-terminal adhesin domains of the PrtR and PrtK have homology with an haemagglutinin from P. gingivalis designated HagA. The gene encoding HagA has been disclosed in the international patent WO96/17936, the disclosure of which is incorporated herein by reference.